listitemmanager: Switch from "insert_before" to "insert_after" argumnet

listitemmanager: Switch from "insert_before" to "insert_after" argumnet

We reorder widgets start to end, so when reusing a list item, we
correctly know the previous sibling for that list item, but not the
next sibling yet. We just know the widget it should ultimately be in
front of.
So we can do a more correct guess of the list item's place in the widget
tree if we think about where to place an item like this.

Actually using this change will come in the next commit.